1. Field of the Invention
This invention relates generally to apparatus and installation methods for semi-submersible vessels, tension leg platforms and floating structures, and more particularly to column-stabilized floating structures having a plurality of vertical spaced apart buoyant caissons connected together by an open framework of truss-pontoon members and the method of installation and operation.
2. Brief Description of the Prior Art
The word truss, as used herein, refers to a welded or bolted open frame structure formed of slender tubular members. Horizontally disposed truss structures are known as xe2x80x9ctruss pontoonsxe2x80x9d, and vertically disposed truss structures are known as xe2x80x9ctruss columnsxe2x80x9d. The truss bridges between three or more buoyant vertical column structures that stabilize a semi-submersible vessel at the water surface when floating with respect to wind, wave, current and other horizontal loads.
Semi-submersible type offshore floating vessels were first introduced in 1950. Ever since their introduction, the semi-submersible vessels have been found to be economical for offshore drilling, production, transport, pipe laying and other shallow and deep-water applications. SPAR, TLP, FPSO""s and barges are other types of floating vessels that can be used for similar applications as semi-submersible vessels. Decks, columns and pontoons are the typical components of a semi-submersible vessel. Semi-submersible vessels comprise three or more large size vertical columns that are spatially separated and connected by large size horizontal pontoons between the columns at their bottoms. The deck structure is above the water with a sufficient air gap between the still water level to the bottom of the deck to allow waves to pass across the columns without impacting on the bottom of the deck. The center of gravity of the entire semi-submersible vessel is generally high due to large deck loads above the water. The deck of a semi-submersible vessel is a very conventional structure made of boxes, trusses and or girders fixed on top of the columns. The modern semi-submersible vessel uses pleated box structures for the deck with bulkheads, frames and stiffeners, in which case, the deck is called the upper hull. The column is a shell type plated structure with stringers, bulkheads and stiffeners.
The pontoon is also a shell type plated structure with frames, bulkheads and stiffeners. The pontoon and columns have compartments for ballast, voids and storages. The column and the pontoon together is called lower hull. The lower hull provides the necessary buoyancy to the semi-submersible to take the functional structural, equipment and live loads. The vertical columns are sized and located apart to provide water plane area and to provide the area moment of inertia. Since the center of the gravity of the semi-submersible vessel is much higher above the center of buoyancy of the lower hull, the acting moments due the wind, wave and currents horizontal forces are restored by the semi column water plane areas and moment of inertia of the water plane areas. The size and the spacing of the columns provide these two stability parameters. Thus the semi-submersible design is called a xe2x80x9ccolumn-stabilized floating unitxe2x80x9d. The semi-submersible vessel is a free-floating vessel compliant to wind, waves and currents. It is moored and/or dynamically positioned by powered-thrusters.
Semi-submersible vessels have good sea-keeping behavior with respect to waves and have large deck areas and carrying capacity. However the current and wind resistance results in higher positioning. A semi-submersible vessel is sensitive to the variable deck loads and its stability is limited. Semi-submersibles have large heave motions (vertical oscillation of the floating vessel) when subjected to waves and consequently a dry-tree oil production system is not feasible. In the case of a drilling semi-submersible, downtime of drilling may occur when the heave motion is not feasible for drilling.
Thomas, U.S. Pat. No. 6,024,040 discloses a mobile jack-up platform converted to a semi-submersible offshore platform. The platform includes single submerged hollow lower base at the bottom end, partially submerged elongate vertical buoyant connecting legs extending upwardly therefrom and passing through an upper barge (jack-up platform) above the level of the sea. The hollow base has a square, rectangular or triangular configuration and is filled with seawater to form the ballast for the entire platform and may include interior reservoirs in which hydrocarbons are stored. A central opening or passage in the center of the base reduces the resistive surface size of the base in the water during vertical movements of the platform. The vertical connecting legs have a hollow cylindrical upper portion with a bottom wall forming a buoyancy tank, and a lower portion formed of open frame lattice construction. The respective lengths of the hollow cylindrical buoyant upper portion and the lattice-work lower portions are dimensioned relative to one another so that a pressure force exerted by the sea on the upper portions substantially compensates for an acceleration force exerted on the base by the action of the seawater surrounding the base over a usual swell period range of the sea. The platform in operation imitates a semi-submersible, which can retract the legs with respect to the upper barge (jack-up platform). With the legs fully retracted above the upper barge and its single lower base closely adjacent to the bottom of the upper barge, the platform can be floated and transported to another location.
The tension leg platform (TLP) was introduced as a new concept based on a semi-submersible design with a deck, columns or caissons and pontoons. The vertical heave of the tension leg platform (TLP) is reduced by tendons attached between the lower hull and the seabed. The tendons are always maintained in tension with the excess buoyancy designed into the lower hull. The application of a TLP is limited to water depths of say 5000 ft., over which, the tendons"" vertical neutral period enters the energetic portion of the wave heave forces of the TLP. Secondly, the size of the lower hull increases the economics to always maintain tension in the tendons with respect to the increased water depth.
The SPAR is currently used in relatively large offshore oil production applications. The SPAR is a single large vertical column or caisson buoyant structure. Unlike the semi-submersibles, the stability of the vessel is not dependent on the water plane area and the moment of inertia of the water plane area. The stability of the SPAR is provided by lowering the center of gravity of the vessel below its center of buoyancy. Thus, the vertical buoyancy acts upwards above the center of gravity, and total weight acts at the center of gravity below the center of buoyancy. The SPAR has excellent heave performance like a TLP and dry-tree production systems like a fixed offshore platform are used. However, the SPAR has limitation in deck area and payload due to the size of the SPAR. The SPAR is fabricated and transported horizontally and up-ended vertically at the operating location. Then the fully equipped deck is installed on top of the SPAR vertical column. Thus the transportation and installation cost and risk are significantly increases with water depth and deck payload applications. The SPAR becomes uneconomical for larger production as a self-contained drilling unit in deepwater. Ultra deepwater posses problems to the riser tensioning system and limits the applicability of the SPAR. Mooring of the SPAR in deepwater over 5000 ft also posses a serious problem. The effectiveness of the mooring and handling are reduced in such water depth. The SPAR also poses vortex shedding vibration problems when the vertical single column hull becomes extremely slender.
Horton, U.S. Pat. No. 5,558,467 discloses a spar-type deep water offshore floating apparatus, for use in oil drilling and production in which an upper buoyant hull of prismatic shape is provided with a passage longitudinally extending through the hull in which risers run down to the sea floor, the bottom of the hull being located at a selected depth dependent upon the wind, wave, and current environment at the well site, which significantly reduces the wave forces acting on the bottom of the hull, a frame structure connected to the hull bottom and extending downwardly and comprising a plurality of vertically arranged bays defined by vertically spaced horizontal water entrapment plates and providing open windows around the periphery of the frame structure, the windows providing transparency to ocean currents and to wave motion in a horizontal direction to reduce drag, the vertical space between the plates corresponding to the width of the bay window, the frame structure being below significant wave action whereby wave action thereat does not contribute to heave motion of the apparatus but inhibits heave motion, the frame structure serving to modify the natural period and stability of the apparatus to minimize heave, pitch, and roll motions of the apparatus. A keel assembly at the bottom of the frame structure has ballast chambers for enabling the apparatus to float horizontally and for stabilization of the apparatus against tilting in vertical position, and taut anchor lines connected to the apparatus at allocation of relatively little cyclic movement of the apparatus, the said lines being connected to suitable anchors. Horton, III, U.S. Pat. No. 5,722,797 discloses a spar-type buoyant floating caisson for offshore drilling and production that includes means for increasing the natural period of the caisson and reducing heave, pitch, and roll without increasing the overall length of the caisson. The floating caisson has a center well through which drilling and/or production risers pass and one or, more circular plates extend radially from the caisson below the water surface. The circular plates provide additional mass and resistance to environmentally induced motions and thus increases the natural period of the caisson beyond the periods of maximum wave energy, which allows the caisson to be designed with a shallower draft than a caisson without the plates that would normally be used in deep water.
Blevins et al, U.S. Pat. No. 6,206,614 discloses a spar-type floating offshore drilling/producing structure that is formed from a plurality of closely spaced vertically oriented buoyant columns on which one or more modules or decks may be placed to support process equipment, a drilling rig, utilities, and accommodations for personnel. The columns are held in the closely spaced relationship by a plurality of horizontal plates spaced along the length of the columns and vertical plates located near the bottom of the columns and near the top of the columns. The columns have a smaller water plane area than the horizontal plates. The structure includes fixed ballast, an oil storage area, and voids and variable ballast for offsetting the lighter weight of the stored oil.
A long-felt need exists for an improved offshore platform that can economically support a deck in deepwater and respond efficiently to the environmental forces in ultra deepwater. An ideal vessel for a deepwater application requires: a large deck area, large carrying capacity, less heave response to waves, excellent wind and wave current motion behavior so that a dry-tree production system is feasible, less station keeping forces, and finally, less construction costs.
The present invention is distinguished over the prior art in general, and these patents in particular by a new concept for offshore drilling, production, transportation and other applications with the above noted key principle requirements for a feasible economical system. The present structure herein referred to as xe2x80x9ccolumn-stabilized floating structures with truss pontoonsxe2x80x9d has a deck and a plurality of vertically oriented buoyant columns or caissons bridged together in distantly spaced relation a plurality of open frame horizontal truss pontoon members and vertical truss columns at a lower end. Each of the buoyant caissons has a submerged portion and a non-submerged portion with one or more buoyant tanks or chambers enclosed by bulkheads or entrapped inside the columns or caissons, the buoyancy of which can be selectively adjusted by means of ballast control. Water is selectively pumped into or out of keel tanks at the bottom of the truss structure such that the water mass and weight is adjustably tuned to raise or lower the center of gravity of the entire mass of the floating structure. By tuning and positioning the center of gravity relative to the center of buoyancy, the present floating structures can be tuned according to ballast and other variable or fixed loads including the deck payloads and compensate for different operational, environmental, and survival conditions and towing and installation stages of the floating structure.
The present column-stabilized floating structures having buoyant caissons with horizontal truss pontoons and vertical truss columns provides a large deck area and deck payload like a conventional semi-submersible, and low heave response to waves and environmental forces like a SPAR vessel. Thus, the present invention has all of the advantages of a semi-submersible and a SPAR vessel. However, unlike the SPAR vessel concept, which requires the center of gravity to be below the center of buoyancy in order to achieve the required stability when floating, the center of gravity of the present structure can be selectively positioned above the center of buoyancy and is stabilized by distantly positioned buoyant caissons, similar to a semi-submersible vessel. By the fundamentals of the principles of the stability criteria, the present invention does not belong to the family of SPAR structures but stabilized by its multiple columns water plane area and the moment of inertia of the water plane area. Spacing the columns well apart increases the moment-of-inertia of the water-plane-area of the water piercing columns. Thus the present invention is called a column stabilized floating structure with truss pontoons.
It is therefore an object of the present invention to provide an offshore column-stabilized floating structure that can economically support a large deck area in deepwater and responds efficiently to environmental forces in ultra-deepwater.
It is another object of this invention to provide a column-stabilized floating structure that has a large carrying capacity and excellent heave response to wind and waves thereby making a dry-tree oil production system economically feasible.
Another object of this invention is to provide a column-stabilized floating structure that is stabilized by buoyant columns or caissons and allows the center of gravity to be above the center of buoyancy in order to achieve stability when floating.
Another object of this invention is to provide a column-stabilized floating structure wherein the center of gravity and weight of the structure can be adjustably tuned as desired to achieve a proper height by pumping water in and out of the keel tanks of the structure.
Another object of this invention is to provide a column-stabilized floating structure that has a low lower hull weight, reduced fabrication cost, and large payload to hull displacement ratio.
Another object of this invention is to provide a column-stabilized floating structure that has low mooring or DP positioning forces, less plated shell type structure and fatigue sensitive spots, and greater residual structural strength than conventional semi-submersible vessels.
A further object of this invention is to provide a column-stabilized floating structure that is insensitive to variable deck loads, allows all equipment and consumable fluids to be located on the deck, and has no risk of damage by dropped objects.
A still further object of this invention is to provide a column-stabilized floating structure that reduces engineering cost, reduces fatigue damages, is economical to manufacture, operate, and maintain.
Other objects of the invention will become apparent from time to time throughout the specification and claims as hereinafter related.
The above noted objects and other objects of the invention are accomplished by a column-stabilized floating structure having a deck and a plurality of vertically oriented buoyant caissons bridged together in distantly spaced relation by a plurality of open frame horizontal truss pontoon members and vertical truss columns at a lower end. Each of the buoyant caissons has a submerged portion and a non-submerged portion with one or more buoyant tanks or chambers enclosed by bulkheads or entrapped inside the columns or caissons, the buoyancy of which can be selectively adjusted by means of ballast control. Water is selectively pumped into or out of the keel tanks at the bottom of the truss structure such that the water mass and weight is adjustably tuned to raise or lower the center of gravity of the entire mass of the floating structure. By tuning and positioning the center of gravity relative to the center of buoyancy, the present floating structures can be tuned according to ballast and other variable or fixed loads including the deck payloads and compensate for different operational, environmental, and survival conditions and towing and installation stages of the vessel.